To facilitate understanding of the present invention, a typical conventional dust collector will be briefly described below with reference to FIG. 1 which schematically illustrates the structure thereof by way of a block diagram. The conventional dust collector identified by reference numeral 1 includes a high voltage power supply source 2 in which an alternate current from a commercial power supply network is rectified to a direct current having high voltage and then the latter is supplied to the dust collector 1 and a control apparatus 3 in which a signal U is generated for the purpose of controlling the direct current having high voltage obtained in said high voltage power supply source 2.
The control apparatus 3 serves for controlling a charging voltage, that is, a direct current having high voltage obtained in the high voltage power supply source 2, for instance, with the aid of a sparking frequency control device or the like incorporated therein so as to keep a dust collecting efficiency at the highest level at all time. In some case where the dust collector has little fluctuation in functional characteristics, a charging voltage is manually controlled so that it is kept constant.
However, in any of the conventional dust collectors, control is effected for a charging voltage so as to keep a dust collecting efficiency at the highest level at all times. Thus, in some cases, there is a necessity for raising up the dust collecting efficiency in excess of a required level, resulting in wasteful consumption of electric power which is undesirable from a viewpoint of energy saving.
In view of the background with respect to the conventional dust collectors as described above, a modified system for collecting dust without a necessity for raising up a dust collecting efficiency higher than a required level was proposed for the purpose of preventing wasteful consumption of electric power. The modified system is schematically illustrated in FIG. 2 in the form of a block diagram and it is constructed such that a dust density measuring device 6 is disposed at the outlet of the dust collector 1 so as to keep a dust density at a constant level at all time by controlling a charging voltage with the aid of the control apparatus 3 adapted to generate a control signal U having an intensity A as illustrated in FIG. 3(A).
In fact, the present invention is concerned with an improvement with respect to the above modified system.